» Niobium Fabrication

The cold working properties of niobium are excellent. Because of its
bcc crystal structure, niobium is a very ductile metal that can undergo
cold reductions of more than 95% without failure. The metal can be easily
forged, rolled or swaged directly from ingot at room temperature.

Annealing is necessary after the cross-sectional area has been reduced
by approximately 90%. Heat treating at 1200° C for one hour causes
complete re crystallization of material cold worked over 50%. Note that
the annealing process must be performed either in an inert gas or in a
high vacuum at pressures below 1 X 10-4 Torr. Of the two methods,
the use of a vacuum is preferred.

Niobium is well suited to deep drawing. The metal may be cupped and
drawn to tube but special care must be taken with lubrication. Sheet metal
can also easily be formed by general sheet metal working techniques. The
low rate of work-hardening reduces springback and facilitates these operations.

» Machining Characteristics

Niobium may be machined using standard techniques. However, due to
the tendency of the material to gall, special attention needs to be given
to tool angles and lubrication.

Niobium also has a tendency to stick to tooling during metal forming
operations. To avoid this, specific lubricant and die material combinations
are required in high pressure forming operations.

» Turning

Machining on a lathe is best performed with high speed steel tools.
For cooling and lubricating, air, soluble oil, Rapid-TapTM or
other suitable products may be used.

The metal turns very much like lead or soft copper. It must be sheared
with the chip allowed to slide off the tool's surface. If any buildup of
material occurs, the pressure will break the cutting edge, ruining the
tool.

Carbide tooling should be used only for fast, light cuts to work efficiently
(.010 to.015 inches deep). Tooling recommendations are given in Table 1.
The data applies to both high speed steel and carbide tools unless otherwise
noted.

» Tooling Recommendations

Approach Angle

15° - 20°

Side Rake

30° - 35°

Clearance:

Side

5°

End

5°

Plan Relief Angle

15° - 20°

Nose radius

0.25" +/- .005"

Cutting Speed: (ft/minute)

High Speed Steel

60 - 80

Carbide

250 - 300

Feed:

Roughing

.010" +/- .002"

Finishing

.005" max.

Depth of cut

.030" - 0.125"

» Drilling

Standard high speed drills, ground to normal angles, may be used. However,
the peripheral lands wear badly so that care must be exercised to ensure
the drill has not worn undersize.

» Screw Cutting

Niobium may be screw-cut using a standard die- cutting head provided
that an ample amount of lubricant is used. The use of sufficient lubricant
prevents galling on the die resulting in the tearing of the thread. Roll
threading is an alternate, and preferred, method.

» Spinning

With some minor modifications, normal techniques of metal spinning
may be applied successfully to niobium.

It is generally better to work the metal in stages. For example, when
spinning a right -angled cup from flat sheet, several formers should be
used to perform the operation in steps of approximately 10'. Wooden formers
may be used for the rough spinning, but a brass or bronze former is essential
for finishing. This is because niobium is soft and readily accepts the
contour of the former.

For small work, aluminum, bronze or narite tools should be used with
a radius of approximately 3/8 inch. Note that if sharp angles are required,
the tool must be shaped accordingly.

Suitable lubrication for this process may be either yellow soap or
tallow, both of which must be continually cold-worked. The peripheral speed
of the work piece should be approximately 500 feet per minute.

Niobium is prone to "thinning" during this process. This
is avoided by working the tool in successive, long, sweeping strokes with
light pressure instead of a few heavy strokes.

» Welding

Niobium is a highly active metal. It reacts at temperatures well below
its melting point with all the common gases, e. g. nitrogen, oxygen, hydrogen
and carbon dioxide. At the melting point and above, niobium will react
with all the known fluxes. This severely restricts the choice of welding
methods.

Niobium can be welded to several metals, one of which is tantalum.
This can be readily accomplished by resistance welding, tungsten-inert
gas, plasma welding and electron beam welding.

Formation of brittle intermetallic phases is likely with many metals
and must be avoided. Surfaces to be heated above 300°C should be protected
by an inert gas such as argon or helium to prevent embrittlement.

It is critical to ensure that the metal is clean prior to welding.
An acid pickle wash is recommended. For ambient temperature pickling, a
typical solution is 25%-35% HF, 25%-33% HN02 with the balance
H20. Coupons should be used before immersing the part to check
the etchant rate. Removal of approximately .0001 inch is generally acceptable.

» Fusion Welding

Niobium can be welded satisfactorily by applying standard gas-tungsten-arc
(G.T.A.W.) heli-arc procedures. The resulting welds are superior to those
made under similar conditions with an alternating current. The argon from
the torch seems to provide better protection for a small pool.

The TIG method is the recommended procedure for welding niobium. However,
some modifications to this method are required.

It is essential to completely cover the area of the molten pool and
the heated zone with inert gas to avoid contamination of the weld metal.
This protection must be given to both the back of the weld and the face.
Several examples of butt joints with different size stock are given below:

<.050 "thick; without filler rod:

For this operation, the torch provides sufficient coverage to the
face of the weld. The back of the weld may be protected with a stream of
argon from a manifold positioned just below the weld bead. A trailing
shield will provide further protection to the hot metal after the main
shield has passed.

<.050" thick, with and without filler rod:

The current required for full penetration now becomes high enough
to cause a spread of the molten pool outside the protection of the argon
shield. The pool also becomes too large for the argon shield when welding
with a filler rod. The solution in this case is to ensure complete
protection with the use of an argon-filled box.

When this metal is exposed to air at reaction temperatures, it acquires
a relatively thick and adherent oxide film that is extremely difficult
to remove.

Vacuum annealing, covered in the section entitled "Welding",
will cause the oxide film to diffuse rapidly into the metal. This results
in a hardening of the weld bead and the heat-affected zone.

Note that contamination-free welds can be produced under totally inert
atmospheres compared to welds produced employing only inert shielding.